Ceramic honeycomb structures, such as those used as catalytic converters and diesel particulate filters (“DPFs”), are manufactured by various processes. Generally, the honeycomb structures are manufactured by extrusion, resulting in a multiplicity of through holes or passages which are separated by the walls of the honeycomb structure. Each passage is sealed at either the inlet or outlet end of the structure and the structure is fired at a high temperature. Adjacent passages are capped alternatively, forming a checkerboard pattern, so that a fluid passing into the structure will be forced to pass through a wall of the structure before passing out of the structure. In this manner, the fluid passing through the structure can either be contacted by a catalyst or particles in the fluid can be filtered, as the fluid passes through the walls of the honeycomb structure.
The catalysts which are used with those honeycomb structures in catalytic converters require high temperatures and high porosity of the honeycomb walls in order to ensure an efficient rate of catalysis. It is therefore necessary that the structure be able to heat up quickly in order to effectively clean exhaust from an engine which has just been started. Those structures which are used as DPFs require that there be low pressure loss as the exhaust passes through the filter, since DPFs are usually utilized in circumstances where the exhaust will pass through the DPF, and then through an independent catalytic converter.
Therefore, it is desired that such honeycomb structures, while being able to withstand the extreme temperatures associated with combustion engines, have a low heat capacity and that the pressure loss through the structure is minimized. In order to achieve these properties, a high porosity and low wall thickness are desirable. However, high porosity and low wall thickness result in low mechanical strength, which results in various problems during production.
In an attempt to rectify these problems, it is now the state of the art to enclose the honeycomb structure within a ceramic paste or mat which will lend the structure increased mechanical strength, protection from vibration, and seal the structure so that, when it is canned, exhaust gases will not pass between the structure and its housing.
It has also been proposed to manufacture multiple smaller honeycomb structures and bond them together using a ceramic adhesive material to create a single structure, which will still require the use of a skin coating around the exterior of the structure to ensure uniformity of the exterior surface. These single honeycomb structures are able to support their own weight more effectively, and the adhesive material lends the structure increased mechanical strength once the monolith is fired.
Whether the monolith is assembled from smaller honeycomb structures or is extruded as a single unit, the exterior of the structure may require machining after the firing step to meet the tight specification tolerances for roundness and actual diameter in the shape of the structure, and to create a surface which will adhere to the skin coating. In some instances, this machining will result in partial honeycomb cells being exposed, which will need to be filled by the skin coating, usually a ceramic paste in these instances.
Skin coatings comprising ceramic pastes are desirable because they can be made of materials similar to that of the ceramic honeycomb structure, resulting in similar heat capacities, and they can be used to perfect the shape of the structure. Mats can be used in conjunction with pastes to provide addition protection from vibration damage to the structure while it is in use. Desirably, the pastes will resist cracking, peeling, and degradation by absorption of acidic catalytic substances. None of the previously proposed ceramic paste materials have sufficiently achieved all of these goals.